Only about 500 million years after the Big Bang, when the universe was roughly 3% of its current age, some of the first stars and galaxies had already begun to shine. Astronomers have generally expected that much of the gas around these young galaxies would still have been nearly pristine, consisting mostly of hydrogen and helium, the basic elements that dominated the newborn cosmos. New evidence suggests that picture may be incomplete.
A study led by astronomers at the University of Arizona and published in Nature Astronomy indicates that early galaxies were already sending heavier elements such as carbon and oxygen into surrounding space much sooner than expected. "We observed that heavy elements escaped from galaxies very, very early in cosmic time," said Yongda Zhu, first author of the paper and postdoctoral researcher at the U of A Department of Astronomy and Steward Observatory. "Not only were the galaxies producing these elements, but they were also dispersing them, possibly seeding other galaxies." The earliest universe contained little besides hydrogen and helium, the two simplest elements at the top of the periodic table.
As gravity gathered this material into dense clouds, stars began to form. Inside those stars, extreme heat and pressure enabled nuclear fusion and other reactions that created heavier elements such as carbon and oxygen. When stars reached the ends of their lives, including some that exploded as supernovae, they released those elements into space.
That material later became part of new generations of stars and planets and eventually contributed to the chemistry needed for life. The carbon in the human body and the oxygen people breathe were both produced inside earlier generations of stars. What remained uncertain was how quickly those heavier elements escaped the first galaxies and spread into the wider universe.
JWST Looks Back More Than 13 Billion Years Zhu and his colleagues focused on three extremely distant galaxies whose light has traveled for more than 13 billion years. The observations show the galaxies as they appeared roughly 500 million years after the Big Bang, during a period called the Epoch of Reionization. During this era, the first generations of stars and galaxies were reshaping the cosmos by ionizing hydrogen gas between galaxies.
In that process, electrons were stripped from hydrogen nuclei. As reionization progressed, ultraviolet light was able to travel more freely through space, helping bring the universe's cosmic "dark ages" to an end. "We used the galaxies themselves as background light sources," said Zhu.
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